In optically sensing information bearing signals recorded in magnetooptic media, two optical detectors are employed. The signals from the two detectors are differentially processed to indicate the recorded information. Optically, the readback from a magnetooptic medium is represented by rotation of the light polarization in the light reflected from the magnetooptic record medium. Using two optical detectors enables an improved detection method; hence the differential electrical signal processing circuits. A problem does arise in such differential processing. Noise in the reflected light beam results from intensity modulation of the light. Such intensity modulation can occur by variations in read beam light intensity caused by unintended changes in read laser operation. Further intensity modulation can occur by unintended magnetooptic medium reflectivity changes, misalignment of optical elements (intensity reduction and the like). Any cause of the intensity modulation of the reflected light beam results in broadband noise in the electrical readback signal. Such reduction in signal-to-noise ratio tends to limit performance of magnetooptic recorders and readers.
Cancellation of the intensity modulation effects on the readback circuits because of misalignment of polarizing optical elements of the readback system and lack of equal amplitudes in intensity modulation induced noise at the two detectors used in the magnetooptic readback system result in incomplete cancellation of common mode noise in the differential amplifier. Therefore, common mode rejection of the noise is not always completely effective. It is desired to automatically and continuously remove intensity modulation induced noise from a magnetooptic readback signal. Such removal is directed at the asymmetrical difference in the amplitudes of such noise at the two magnetooptic detectors.